QUARTIC ANHARMONIC RESONANCES IN ACETYLENES AND HALOACETYLENES

Formulas are derived for the quartic anharmonic resonance coefficients observed to be important between C-H stretching and the combination of one quantum of C≡C stretching and two quanta of H-C≡C bending in a number of acetylene molecules. Examples of this resonance are ν3 with ν2+ν4+ν5 in 12C 2H2, ν1 with ν2+2ν 5 in 13C2H2, and ν1 with ν2+2ν4 in monofluoroacetylene and monochloroacetylene. The coefficients characterizing the resonances in these examples, which we denote K3,245, K1,255, and K 1,244, arise from cubic and quartic terms in the anharmonic force field, in the normal coordinate representation, through second order and first order perturbation treatments respectively, where the second order resonances are calculated by a Van Vleck resonance formalism. The experimentally determined values of these coefficients are compared with values calculated from model anharmonic force fields. © 1995 American Institute of Physics.